The present invention relates to a piezoelectric resonator employing a piezoelectric ceramic material, and more specifically to a piezoelectric resonator which can be used as an oscillator for electronic timekeeping devices and communications systems, or which can be used as an ultrasonic device for ME systems.
The piezoelectric resonator consists of a piezoelectric plate with metal electrodes formed on both surfaces, and works based upon the principle of interconversion between electric vibration and mechanical vibration.
So far, the piezoelectric plate used for the piezoelectric resonators has been made of a ceramic material of the so-called PZT type consisting of lead zirco-titanate (Pb(Zr.sub.1-x Ti.sub.x)O.sub.3) as a main component, or piezoelectric quartz crystals. The PZT-type ceramics, however, have high dielectric constants, and input/output impedances become very low when they are used under high-frequency conditions, presenting a problem of impedance matching with the external circuits. In particular, it is the modern tendency to use the piezoelectric resonators under high-frequency conditions, and it has been desired to develop piezoelectric resonators having better temperature characteristics and resonance properties so that they can be used under high-frequency conditions. As a promising example of the piezoelectric material of a resonator for use under high-frequency and high-temperature conditions, attention has been given to a ceramic material of the type of lead titanate (PbTiO.sub.3) composed chiefly of lead oxide (PbO) and titanium oxide (TiO.sub.2) owing to its low dielectric constant and high Curie temperature. In recent years, it has been discovered that poor temperature characteristics inherent in the PbTiO.sub.3 -type ceramics can be greatly improved by the addition of neodymium oxide Nd.sub.2 O.sub.3, manganese oxide MnO.sub.2 and the like (U.S. Pat. No. 4,243,541). This fact brings about the probability that the PbTiO.sub.3 -type ceramics will find extended fields of applications. Due to its small Poisson ratio, however, the PbTiO.sub.3 -type ceramics are not capable of trapping the vibration energy by lowering the frequency beneath a portion of the electrode, unlike the PZT-type ceramics. When used as a resonator or a filter based upon the thickness dilatational mode over a frequency range of several megahertz to several tens of megahertz, therefore, the PbTiO.sub.3 -type ceramics generate spurious signals and thus make it difficult to obtain a high Q value. In recent years, a variety of methods have been proposed to realize trapping of the vibration energy by increasing the frequency in order to improve the resonance properties of the PbTiO.sub.3 -type ceramics. However, these ceramic resonators have yet to be put into use.